1. Field of the Invention
This invention relates to optical glass and, more particularly, the invention is directed to a composition of optical glass having refractive index (nd) of from 1.71 to 1.74 and Abbe number (v) of from 49 to 54 as the optical constants thereof.
2. Description of the Prior Art
For optical glass having a refractive index of from 1.71 to 1.74 and Abbe number of from 49 to 54, there is known a glass which consists principally of B.sub.2 O.sub.3 as the network forming oxide of the glass, La.sub.2 O.sub.3 as the network modifying oxide which imparts to the glass the optical characteristics of a high refractive index and low optical dispersion, and a divalent metal oxide (RO), for example, CaO, SrO, BaO, ZnO, CdO or PbO as another network modifying oxide having the function of a flux. (An optical glass, when its melting temperature is higher, has some disadvantages of devitrification and of worse workability. The flux serves, when mixing the glass, to prevent the melting temperature of glass from being higher as leading to the above disadvantages. Different ingredients to be mixed need different fluxes.) A glass composition of this type is disclosed in U.S. Pat. No. 3,081,178. The optical glass of this composition, however, is limited in its chemical durability, because it does not contain a sufficient quantity of a divalent metal oxide (RO) so as to be stable within its optical constants.
In an attempt to overcome the above-described short-coming, it has been proposed to provide an optical glass having the above-mentioned optical constants and which is made without the divalent metal oxide (RO). In U.S. Pat. No. 3,174,871 a composition is disclosed which consists of, by weight, 6 to 36% of B.sub.2 O.sub.3, 7 to 37% of SiO.sub.2, 50 to 67% of La.sub.2 O.sub.3, and 7 to 21% of Al.sub.2 O.sub.3. The optical glass of this composition, however, contains a remarkably large quantity of La.sub.2 O.sub.3, as high as 50% and higher, and as a result, the temperature required for its melting is extremely high; also, it has a tendency to devitrify. In view of these limitations, this type of optical glass is not commercially practical.